In a control apparatus for an AC rotary machine wherein the variable speed control of the AC rotary machine is performed by a power inverter without employing any rotational angular velocity detector for the AC rotary machine, a cost and wiring which are required for the rotational angular velocity detector can be omitted. Since, however, the rotational angular velocity detector is not employed, the rotational velocity of the AC rotary machine cannot be known when the AC rotary machine starts in a free-run state, that is, in a state where the power inverter stops a power inversion operation and where the AC rotary machine is rotating at any desired rotational velocity, so that a stable start is difficult to be performed without generating large fluctuations in the torque and rotational velocity of the AC rotary machine.
A start method wherein an induction motor is started without employing any rotational angular velocity detector is disclosed in JP 63-77397A (Patent Document 1). In Patent Document 1, the instantaneous magnetic flux vector signal and instantaneous generation torque signal of the motor are calculated using switch state signals for commanding the respective phase output voltages of a power inverter, the voltage detection value of a DC voltage source and the current detection value of the motor, and three control flags are generated using the instantaneous magnetic flux vector signal and instantaneous generation torque signal of the motor. Owing to the combination of the three control flags, the switching state of the power inverter is designated so as to generate a voltage vector optimizing a torque response, the generation torque of the motor is controlled so as to follow up a command value, and a magnetic flux vector is controlled so as to depict an approximate circular locus, so that the induction motor is started from a free-run state. In the start control, the angular frequency of an AC voltage in the power inverter is set higher than the highest angular frequency which arises in a normal operation, and a switch 4 on the DC input side of the power inverter is thereafter closed to pull in the motor.
Patent Document 1: JP 63-77397A